1. Field of the Invention
The invention relates to the setting of thresholds for handoff of mobile stations within a cellular radio system and, more particularly, to a method and apparatus for self-tuning a parameter indicating the minimum, sufficient signal strength threshold used in determining whether or not a preferred cell is suitable for communication in a hierarchical cell structure of, e.g., macrocells, microcells and picocells.
2. Related Art
In cellular radio communication systems a geographic area is divided into a plurality of individual regions referred to as cells. Each of these cells is provided with radio service on a plurality of separate RF channels the frequencies of which are reused among different cells. The different cells are separated from one another that there is a sufficiently low level of interference between them. When a mobile station receiving radio service from a particular base station serving a particular cell moves from that cell into another adjacent cell communication with the mobile station is xe2x80x9chanded offxe2x80x9d from the first base station to the second base station serving the adjacent cell. Such handoff is typically accomplished by a mobile switching center (MSC) to which all of the base stations are connected and which controls the allocation of communication channels to the individual mobiles moving throughout the region served by the cells.
As the demand for cellular radio service increases over time, the capacity of existing systems has been severely stressed to serve all the subscribers who would like to have access to the system, particularly in major metropolitan areas. To address this demand, cellular radio technology is moving to digital based systems such as time division multiple access (TDMA) in which a plurality of subscriber channels can be assigned to each radio channel. In a TDMA system, each channel is divided by time slots, such that information is transmitted in the same frequency but at different times as other radio signals. The received signals are separated or demodulated according to appropriate decoding.
However, even with such improvements in channel capacity, there exist certain areas within major metropolitan areas in which the demands on the system are so great that it cannot be successfully satisfied by existing cellular radio architectures. For example, in the area in and around a convention center located in a major metropolitan area, the channel usage by portable cellular radio transceivers may be so great that the demands for service cannot be satisfied by the entire channel capacity of the base station serving the cell within which the convention center is located. In such situations, it has been proposed to provide additional xe2x80x9clayersxe2x80x9d of cellular radio coverage provided by additional lower powered base stations located within an existing, so-called xe2x80x9cumbrellaxe2x80x9d cell and referred to as xe2x80x9cmicrocellsxe2x80x9d. Such microcells may have a coverage or service area on the order of a few hundred meters in contrast to a few kilometers of coverage by the base station of the overlying umbrella cell. A plurality of such microcells may be located adjacent to one another and form a contiguous coverage area of substantial width all of which is within the overall coverage area of the umbrella cell.
When a layered cell structure, as described above in conjunction with umbrella cells and microcells, is used there is provided an enhanced level of radio capacity which can be configured for individual circumstances and which provides an assurance that users can receive service despite an extremely high demand within a small geographic area. Moreover, additional layers of radio coverage may be added, for example, by a plurality of either contiguous or separated picocells positioned within the coverage or service area of the individual microcells, each of which are in turn within the overall umbrella cell. The base stations providing the radio coverage within the picocells would be of even still lower power than the base stations serving the microcells and have a coverage or service area of, for example, a hundred meters to provide coverage within a single building or a single floor within a large convention center.
Thus, in layered cell architectures, the issue of base station server selection and handoff of each mobile radio transceiver moving within a geographic area involves many more options. That is, it is possible for the mobile station to receive radio service at any given moment from either a picocell base station, a microcell base station, or an umbrella cell base station. When conventional handoff criteria designed for use in single layered cellular architectures are applied to this situation, problems arise and the solution is less than ideal. The ability to configure the handoff mechanism for maximum efficiency with respect to the utilization of channel availability and consistent with high quality radio service to each mobile subscriber is highly desirable.
When handoff is effected between adjacent cells in a single layer cellular radio architecture, the principle criterion used is the comparison of the quality of the signal received from the mobile station by the respective base stations capable of providing radio service. Alternatively, the mobile station may perform measurements on signals received from the base station of the serving cell and from base stations of adjacent cells and report the results to the serving base station. That is, the signal quality of the serving cell is compared with the signal quality of an adjacent cell and when the quality of signal in the latter exceeds the former, the mobile station is handed off to the base station serving the adjacent cell. In addition, a signal quality increment, known as an offset or hysteresis, is also applied to the signal quality difference value so that unless the signal quality in the adjacent base station is at least xe2x80x9cxxe2x80x9d amount greater than the presently serving base station, handoff does not occur. This prevents oscillating handoffs due to signal quality perturbations in which the mobile is repeatedly handed back and forth between two adjacent base stations.
As disclosed in U.S. Pat. No. 5,499,386 issued to Bror Karlsson, (herein incorporated by reference) to optimize service quality and capacity, it is preferable to serve mobile stations with the lowest possible level of base station transmission power while assuring that sufficient signal quality exists. Hence, there is generally a preference to serve mobile stations in pico or microcells, rather than umbrella cells which has fewer total channels available for service than the plurality of adjacent microcells would have. Pico and microcells are thus defined as xe2x80x98preferredxe2x80x99 cells and umbrella cells as xe2x80x98non-preferredxe2x80x99 cells. The Karlsson patent introduces a system of handoff algorithms which maximize the efficient utilization of channel availability within a multi-level or hierarchical cellular radio architecture. In Ericsson""s CMS8800 system, a mobile station is served by the base station of a cell belonging to the preferred level as long as the measured signal strength, which is closely related to the signal quality of that particular cell, is above a selected minimum value, i.e. the xe2x80x9csufficient signal strengthxe2x80x9d parameter SSSUF. The sufficient signal strength parameter is used to define the lowest signal strength level at which the preferred cell will be prioritized over a non-preferred cell.
Within a certain range of interference and noise in a cell, the setting of the sufficient signal strength parameter will determine the call quality in the cell.
However, the setting of these parameters is done manually in conventional systems such as Ericsson""s CMS8800 system which utilizes the concept of hierarchical cell structure to force traffic from non-preferred cells into preferred cells even though the non-preferred cell may provide a higher signal strength.
The tuning of this and other parameters for microcells and picocells is time consuming and often inaccurate. In a congested network, there may be thousands of micro and picocells each serving a fairly small amount of traffic. Considering the relatively small returns in subscriber fees for each of these cells, it is often not cost effective to spend too much time in fine tuning the parameters of every single cell. To minimize this work, a common parameter setting is often used for all microcells or picocells. The selection of such parameters is often a trade-off between quality and capacity. Since the parameters are not tuned individually in each cell, the handoff mechanism may be satisfactory in some cells and not so good in others. There may thus be call quality problems in some cells, e.g., if the level of co-channel interference on the frequencies used in the microcell is high due to short re-use distances. In that case, a relatively high value of SSSUF is needed. On the other hand, if a high value of SSSUF is used in general to safeguard quality, the usage of the micro and picocells, i.e. preferred cells, will decrease, thus limiting the overall system capacity. In the best case, system measurements are conducted to set such parameters as the sufficient signal strength parameter.
In one aspect of the invention, the parameters used to define the lowest signal strength level at which a preferred cell will be prioritized over a non-preferred cell is automatically tuned based on a signal quality criteria. For instance, the signal quality criteria can be speech quality.
Specifically, the present invention includes a method of determining a minimum acceptable, i.e., sufficient, signal strength in a cell of a hierarchical cell structure comprised of multiple levels of cells having different service areas and including base stations and mobile stations. The method comprising the steps of: 1) assigning to each cell according to a cell level a category of preference for selection with respect to each other associated cell within the system having either a coextensive, adjacent, contiguous, or overlapping service area; 2) assigning to each associated cell a preselected sufficient signal strength threshold; and 3) at least initially adjusting said threshold in accordance with a measured quality criteria.
In the exemplary embodiment the method measures the speech quality by a bit error rate (BER). In accordance with this method the adjusting of the preselected sufficient signal strength threshold includes the following algorithm:
If BER greater than 1% for more than 2% of the call time in a cell, the preselected sufficient signal strength threshold is increased by 1 dB; and
if BER greater than 1% for less than 0.5% of the call time in this cell, the preselected sufficient signal strength threshold is decreased by 1 dB.
Alternatively, the method can adjust the preselected sufficient signal strength threshold with a non-recursive, statistical relationship estimate of how much the signal strength threshold should be increased or decreased, such as by determining the probability of a bit error rate being greater than 1% for a given threshold level and determining a threshold level to be increased or decreased to assure that that probability is within a given range.
The present invention can be embodied as a method of handoff of a mobile station operating within a cellular radio system in a hierarchical cell structure comprised of multiple levels of cells having different service areas and including base stations and mobile stations, the method comprising the steps of: 1) assigning to each cell according to a cell level a category of preference for selection with respect to each other associated cell within the system having either a coextensive, adjacent, contiguous, or overlapping service area; 2) assigning to each associated cell a preselected sufficient signal strength threshold; 3) at least initially adjusting the preselected sufficient signal strength threshold in accordance with a measured quality criteria; 4) measuring the signal strength of communications between the mobile station and each base station serving the associated cells; 5) comparing the measured signal strength to the preselected thresholds for each of the measured cells; 6) and selecting for the mobile station a base station for a communication based upon whether the measured signal strength of the base station is greater than the preselected sufficient signal strength threshold assigned to the cell and whether the base station has a higher category of preference than the currently serving cell.
Further, the present invention can be embodied in an apparatus for determining a minimum acceptable signal strength in a cell of a hierarchical cell structure comprised of multiple levels of cells having different service areas and including base stations and mobile stations, the apparatus comprising: 1) means for assigning to each cell according to a cell level a category of preference for selection with respect to each other associated cell within the system having either a coextensive, adjacent, contiguous, or overlapping service area; 2) means for assigning to each associated cell a preselected sufficient signal strength threshold; 3) means for at least initially adjusting the preselected sufficient signal strength threshold in accordance with a measured quality criteria.